[feature] Create arbitrary plot legends

[ewels]

It'd be nice to be able to supply a totally custom legend for a plot. For example, in cases where large scatter plots have thousands of points grouped into a handful of categories (eg. somalier), having a legend of those categories would be very helpful.

As the use cases here could vary, my suggestion would be to simply supply names and colours. Not sure if Plotly will allow this.

See https://community.seqera.io/t/adding-legend-to-somalier-pca-plots/637 for context.